In recent years, there have been proposed various photodetectors including, for example: the one having a quantum well structure (see, for example, Non-patent Literature (NPL) 1); the one having, in place of the quantum well, an infrared absorber using a metal (Au)/semiconductor material (Ge) (metamaterial type) (see, for example, NPL 2); the one utilizing a metal-semiconductor junction using silicon (see, for example, NPL 3); and the one having antenna layers for generating surface plasmon resonance (see, for example, Patent Literature (PTL) 1).